One of the most accurate ways of measuring frequency is with a frequency counter which displays numerically the frequency of the signal applied to its input. Frequency counters generally employ a crystal oscillator for generating a reference frequency against which the frequency of the input signal is compared. Consequently, the accuracy of the frequency counter is dependent on the accuracy of the reference frequency and thus on the accuracy of the reference or master crystal oscillator.
Since the oscillator frequency tends to change in response to changes in the ambient temperature the crystal oscillators of precision frequency counters have been mounted in constant temperature ovens which maintain the crystal oscillators at constant temperatures irrespective of ambient temperature changes within a wide range. A disadvantage of such temperature controlled devices is the large amount of power which they consume. Consequently, high accuracy, portable frequency counters are rare, and those that are on the market have short battery lives.